The present invention relates to apparatus which generates an audio signal from speech and which is of assistance to the profoundly deaf when lipreading.
As mentioned in "Speech Pattern Element Stimulation in Electrical Hearing" by Adrian Fourcin, Ellis Douek, Brian Moore, Evelyn Abberton, Stuart Rosen and John Walliker, Archives of Otolaryngology, Vol. 110, March 1984, the profoundly deaf may be assisted in lipreading if larynx frequency information is presented to them acoustically in pure sine wave form. The explanation for this finding is not certain, but may be that a sinusoid is the only waveform which can pass through a dispersive linear filter (transducer, ear canal, tympanic membrane, ossicular chain [if present], cochlea) and still retain its overall shape.
A major problem which is encountered in producing the required sine wave output is the generation of an accurate sine wave of varying amplitude over a frequency range from about 30 Hz to about 800 Hz.